Hen with chick

I had one of my specled sussex hens hatch just one chick. I finally found her hiding spot and moved them into a spare stall. The chick is doing great and will 3 weeks old this weekend. When can I put her back in with the flock? Some her friends sit outside looking at her as I think they having her as a buddy. I just don't want anything to happen to the little buggar. I was thinking about building them a pen inside the regular coop for a few days and see how that goes. Then letting them out if all goes well.